The present invention relates to printing plate materials suitable for imaging by digitally controlled laser radiation. More particularly, the invention relates to printing plate materials having one or more layers of an organic composition thereon.
Printing plates suitable for imaging by digitally controlled laser radiation include a plurality of imaging layers and intermediate layers coated thereon. Laser radiation suitable for imaging printing plates preferably has a wavelength in the visible or near-infrared region, between about 400 and 1500 nm. Solid state laser sources (commonly termed xe2x80x9csemiconductor lasersxe2x80x9d) are economical and convenient sources that may be used with a variety of imaging devices. Other laser sources such as CO2 lasers and lasers emitting light in the visible wavelengths are also useful.
Laser output can be provided directly to the plate surface via lenses or other beam-guiding components, or transmitted to the surface of a blank printing plate from a remotely sited laser through a fiber-optic cable. A controller and associated positioning hardware maintains the beam output at a precise orientation with respect to the plate surface, scans the output over the surface, and activates the laser at positions adjacent selected points or areas of the plate. The controller responds to incoming image signals corresponding to the original figure or document being copied onto the plate to produce a precise negative or positive image of that original. The image signals are stored as a bitmap data file on the computer. Such files may be generated by a raster image processor (RIP) or other suitable means. For example, a RIP can accept data in page-description language, which defines all of the features required to be transferred onto a printing plate, or as a combination of page-description language and one or more image data files. The bitmaps are constructed to define the hue of the color as well as screen frequencies and angles.
The imaging apparatus can operate on its own, functioning solely as a platemaker, or can be incorporated directly into a lithographic printing press. In the latter case, printing may commence immediately after application of the image to a blank plate, thereby reducing press set-up time considerably. The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the lithographic plate blank mounted to the interior or exterior cylindrical surface of the drum. Obviously, the exterior drum design is more appropriate to use in situ, on a lithographic press, in which case the print cylinder itself constitutes the drum component of the recorder or plotter.
In the drum configuration, the requisite relative motion between the laser beam and the plate is achieved by rotating the drum (and the plate mounted thereon) about its axis and moving the beam perpendicular to the rotation axis, thereby scanning the plate circumferentially so the image xe2x80x9cgrowsxe2x80x9d in the axial direction. Alternatively, the beam can move parallel to the drum axis and, after each pass across the plate, increment angularly so that the image on the plate xe2x80x9cgrowsxe2x80x9d circumferentially. In both cases, after a complete scan by the beam, an image corresponding (positively or negatively) to the original document or picture will have been applied to the surface of the plate.
In the flatbed configuration, the beam is drawn across either axis of the plate, and is indexed along the other axis after each pass. Of course, the requisite relative motion between the beam and the plate may be produced by movement of the plate rather than (or in addition to) movement of the beam.
Regardless of the manner in which the beam is scanned, it is generally preferable (for reasons of speed) to employ a plurality of lasers and guide their outputs to a single writing array. The writing array is then indexed, after completion of each pass across or along the plate, a distance determined by the number of beams emanating from the array, and by the desired resolutions (i.e., the number of image points per unit length.)
Some prior art patents disclosing printing plates suitable for imaging by laser ablation are Lewis et al. U.S. Pat. Nos. 5,339,737, 5,996,496 and 5,996,498.
Although these prior art printing plates perform adequately, certain of them are expensive to produce because the absorbing layer is vapor deposited onto an oleophilic polyester layer. Adhesive bonding of the polyester layer to a metal substrate also adds to the cost.
The present invention includes a printing plate material having a substrate coated with one or more layers of a polymer composition. The substrate may be a metal, preferably an aluminum alloy or steel, paper or plastic.
In one embodiment, a laser-ablatable member including a polymeric composition is positioned on one side of the substrate. When the substrate is metal, the principal surface may be finished by at least one of roll texturing, mechanical texturing, chemical texturing or electrochemical texturing. The laser-ablatable member preferably is formed from a polymer composition including a hydrophilic acrylic polymer and a plurality of laser-sensitive particles, wherein the polymer composition is ablatable when a laser irradiates the laser-sensitive particles. A preferred acrylic polymer is a copolymer containing an organophosphorous compound, particularly, a copolymer of acrylic acid and vinyl phosphonic acid. The laser-sensitive particles preferably are dyes, metals, minerals or carbon. The laser-ablatable member may be formed from an oleophilic thermoplastic or elastomeric polymer wherein an upper portion of the laser-ablatable member is treated to be hydrophilic.
A portion of the laser-ablatable member includes a layer not having the laser-sensitive particles. The layer not having laser-sensitive particles has a different affinity for a printing liquid from a remainder of the laser-ablatable member having the laser-sensitive particles. This layer may underlie the remainder of the laser-ablatable member, overlie the remainder of the laser-ablatable member or be positioned intermediate of the remainder of the laser-ablatable member. When the layer not having the laser-sensitive particles underlies the laser-ablatable member, the underlying layer may include a plurality of insulating particles such as particles of barium sulfate, titanium dioxide, alumina or silica or combinations thereof. The insulating particles block heat generated by irradiation of the laser-sensitive particles in the laser-ablatable member from passing to the substrate.
Alternatively, a portion of the laser-ablatable member may include a second polymer having a different affinity for printing liquid from the polymer composition. Suitable second polymer compositions include an acrylic polymer without the laser-sensitive particles, a silicone polymer or a thermoplastic or elastomeric polymer.
In another embodiment of the invention, the printing plate includes a substrate, a first layer comprising a first polymer composition overlying the substrate and a second layer comprising a second polymer composition overlying the first layer, wherein the first layer and second layer have different affinities for a printing liquid. The first polymer composition includes an acrylic polymer and includes a plurality of laser-sensitive particles. The second polymer composition may include a hydrophilic polypropylene composition, an acrylic polymer or a silicone polymer or copolymer. Preferably, the acrylic polymer is a copolymer of acrylic acid and vinyl phosphonic acid. The printing plate may further include a third layer underlying the first layer. The third layer is formed from a hydrophilic polypropylene composition, an acrylic polymer or a thermoplastic or elastomeric polymer. The third layer may be applied to the substrate via roll coating, spray coating, immersion coating, emulsion coating, powder coating or vacuum coating. Alternatively, the third layer may be a conversion coating of a salt of or a compound of Zn, Cr, P, Zr, Ti or Mo or it may be formed of an epoxy resin electrocoated onto the substrate.
In yet another embodiment of the invention, imaging radiation does not cause ablation of any polymer layer. This embodiment includes a printing member positioned on the principal surface of the substrate and having an upper surface formed from a polymeric composition that is non-ablatable by imaging radiation. The upper surface has an initial affinity for a printing liquid and is changeable to a different affinity for a printing liquid when the printing member is subjected to imaging radiation. The polymeric composition preferably includes an acrylic polymer; more preferably includes an organophosphorous compound. The printing member may include a first layer underlying the upper surface. The first layer is formed from a polymer, preferably an acrylic polymer, and a plurality of radiation-absorbing particles such a dye, a metal, a mineral or carbon. A second layer may underlie the first layer and may be an acrylic polymer or a conversion coating of a salt or compound of Zn, Cr, P, Zr, Ti or Mo. Alternatively, the printing member may have an upper surface, which is ablatable by imaging radiation to expose underlying polymer. The imaging radiation causes the affinity to a printing liquid of the underlying polymer exposed during ablation to change to a different affinity to a printing liquid.
A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout.